The present invention relates to a transparent conductive film of high transmittance in the visible light region and the infrared region, with low film resistivity, and in which the crystallization temperature can be controlled, as well as the manufacturing method thereof, and an oxide sintered compact for use in producing such transparent conductive film.
As a transparent conductive film, tin-doped indium oxide (hereinafter referred to as “ITO”) is being broadly used as an electrode material of an FPD (flat panel display) and the like since it possesses superior characteristics such as low resistivity and high transmittance.
Nevertheless, since ITO has a high carrier concentration and inferior transmittance in a long wavelength region, it is not necessarily effective as a transparent electrode for solar batteries which have been making remarkable progress in recent years. This is because, since the spectral sensitivity of solar batteries is up to approximately 1200 nm for crystalline silicon models and up to approximately 1300 nm for CIGS (Cu—In—Ga—Se-based) models, high transmittance is required up to this kind of long wavelength region.
Under the foregoing circumstances, zirconia-doped indium oxide (hereinafter referred to as “IZrO”) has been proposed as a candidate material with high transmittance even in a long wavelength region and with low resistivity to replace ITO. Since IZrO has high mobility and low carrier concentration, it is attracting attention since the transmittance could be maintained relatively high even in a long wavelength region.
The following documents have reported findings related to IZrO. Japanese Patent Laid-open Publication No. H6-160876 describes zirconium-doped indium oxide. Nevertheless, the description is limited to presenting zirconium-doped indium oxide as a low resistivity material that can be substituted for ITO, and the additive to be added to indium oxide was merely switched from tin to zirconium. Although the Examples describe that the resistivity of the obtained film is extremely low, since the carrier mobility is extremely high at 1021 cm−3, there is no choice but to say that the transmittance in the long wavelength region is extremely low and inferior as with ITO. Only one type of result is described regarding the zirconia concentration, and there is no description regarding an appropriate zirconia concentration. With respect to the substrate temperature during the deposition, the only descriptions that are provided are that annealing is performed at 220° C. after deposition at 250° C. and room temperature deposition, and there is no description concerning the crystallinity of the film. In addition, there is no technical concept of controlling the etching rate by using the crystallinity or controlling the crystallization temperature. Although Japanese Patent Laid-open Publication No. H6-160876 describes that the target used in the sputtering is of “high density”, no description considering the specific value thereof is found. Furthermore, there is no description concerning the bulk resistance. With respect to the gas during sputtering which considerably affects the electrical properties of the film, it is only described as “mixed gas in which trace amounts of oxygen gas was added to argon gas.”
Japanese Patent Laid-open Publication No. 2002-226966 and Japanese Patent Laid-open Publication No. 2002-373527 describe zirconium-doped indium oxide. Nevertheless, the resistivity of the amorphous film during the room temperature deposition is high, while there is no description concerning the resistivity of the film during the deposition at 200° C. Moreover, the concept of controlling the crystallization temperature of the film based on the type or concentration of the additive cannot be acknowledged. Although the density of the oxide sintered compact as the sputtering target is relatively high, the highest relative density that is described is 98.7%, and a target with even higher density is required in order to inhibit the nodules that are generated after prolonged sputtering.
Japanese Patent Laid-open Publication No. 2007-273455 describes that a zirconium-doped film having indium oxide as its main component yields superior electron mobility and specific resistance as an oxide transparent conductive film with high transmittance in the long wavelength region. Nevertheless, the substrate temperature in the Examples is extremely high at 650° C., or 450° C. at the lowest, but as a practical issue, there are significant restrictions in the actual use unless the substrate temperature is at least 300° C. or less. This is because there will be restrictions regarding the substrate material that can be used, and because it is necessary to appropriately maintain the electron concentration profile at the p-n interface of solar batteries. With respect to a zirconia-doped indium oxide target, there is no description concerning the properties such as the sintered compact density, the bulk resistance and the like. With respect to this point, it is assumed that the bulk resistance of the sintered compact that was used as the target was high since RF sputtering was adopted in the Examples rather than DC sputtering.
The Journal of the Surface Science Society of Japan, Vol. 29, No. 1, pages. 18-24, (2008) and the Journal of Applied Physics, Vol. 101, No. 063705, (2007) describe zirconia-added indium oxide. Nevertheless, the subject matter thereof shows that the substrate temperature is extremely high as with Japanese Patent Laid-open Publication No. 2007-273455, there is no description concerning the density of the sintered compact that was used as the target, and, as with Japanese Patent Laid-open Publication No. 2007-273455, RF sputtering is performed.
As described above, with respect to a zirconia-doped indium oxide sintered compact, an oxide sintered compact with sufficiently high density and low bulk resistance of a level required industrially did not exist to date. A scheme of controlling the crystallization temperature based on the type or concentration of the additive to enhance the etching rate of the film obtained by sputtering deposition using such oxide sintered compact as the sputtering target was not adopted.